This invention relates to a sleeve-like sealing unit, particularly an extender for a thimble guide in a nuclear power plant.
In a conventional pressurized water reactor vessel, in the lower portion thereof, a horizontal lower core plate extends, whose top face supports a large number of fuel assemblies each having a bottom nozzle. The bottom nozzle has a horizontal platform and spaced legs which extend downwardly therefrom and which stand on the lower core plate.
A number of measuring instruments are employed to promote safety and to permit proper control of the nuclear reaction. Among other measurements, a neutron flux map is generated periodically, for example, every 28 days, using data gathered by neutron flux detectors which are moved through a number of selected fuel assemblies. To guide the flux detectors during their periodic vertical travel parallel to the length dimension of the fuel rods of the fuel assemblies, stainless steel tubes known as flux thimbles extend through the bottom of the reactor vessel and into the selected fuel assemblies. Aligned vertical channels are provided in the reactor vessel wall, the lower core plate as well as the platform of the bottom nozzle of each selected fuel assembly to accommodate the thimble which thus vertically extends from the outside of the reactor vessel through the aligned passages to the top of the fuel assembly. The flux detector is inserted into the thimble and moved therein along the fuel assembly while the detected flux is recorded. During refuelling the thimbles are withdrawn from the reactor vessel and subsequently driven back into the fresh fuel assemblies.
For accurately guiding, supporting and stabilizing the thimble, into the thimble-receiving vertical passage of the lower core plate there is installed, at the top of the lower core plate, a thimble guide which is essentially a rigid sleeve member projecting from the top face of the lower core plate vertically towards the platform of the associated fuel assembly bottom nozzle and in which the thimble may slide. The upper terminus of the thimble guide is situated at an appreciable distance from the bottom face of the nozzle platform to prevent any abutting engagement between the thimble guide and the nozzle. Such an engagement, due to the rigid construction of both the thimble guide and the nozzle platform would adversely affect the stable footing of the bottom nozzle, sought to be ensured by an engagement between the bottom face of the nozzle legs and the top face of the lower core plate.
Due to the above-discussed spacing between the thimble guide and the underside of the nozzle platform, the thimble portion which bridges that distance is, in the absence of additional shroud arrangements, directly exposed to the coolant flow in the reactor vessel. Since considerable coolant turbulence exists during operation of the reactor in the region between the upper surface of the core plate and the lower surface of the lower nozzle platforms of the fuel assemblies, such turbulence may cause undesired vibration and wear of the thimbles. These phenomena are enhanced by the fact that, in the absence of a shielding arrangement, the coolant flowing upwardly through the thimble channel in the core plate and the thimble guide does not have a well-defined flow path leading into the thimble passage in the nozzle platform.